In the type of rotor called jet-driven rotor, the driving force of the rotor is created by the expansion and ejection of pressurized gas and not by a mechanical device connected to the fuselage of the aircraft. Such an aircraft has two contra-rotating rotors, the gyroscopic momentums of which cancel one another out, which makes it possible to avoid using a tail rotor. Piloting is simplified and the reliability of the aircraft is increased.
It is well known that the variations in lift and traction provided by a primary helicopter rotor are ensured on the one hand by uniformly modifying the angle of attack (or the pitch) of the blades owing to the collective pitch control and on the other hand by modifying the pitch of each of the blades individually and cyclically, owing to the cyclic pitch control.
The essential element allowing the pilot to transmit the collective pitch and cyclic pitch controls to the blades of the primary rotor is the swash plate.
According to one example swash plate among others in the prior art, the latter comprises two annular plates assembled by a ball bearing and mounted concentrically around the shaft of the primary rotor, which is driven in rotation by the motors via the gearbox and ends with a hub on which the blades are articulated. The two superimposed plates slide and roll around the shaft of the rotor. The lower plate is stationary relative to the structure of the helicopter and the upper plate rotates with the rotor. Pitch links, each connected to a blade and varying its angle of attack, are fastened on the upper plate. The lower plate, tilted and/or moved along the axis of the rotor by the controls, owing to a ball joint with bearings, transmits these movements to the upper plate to which it is connected.
The collective (or general) pitch control then corresponds to an axial translation of the set of the two plates, obtained by axial sliding of the articulation mechanism along the shaft of the rotor and the cyclic pitch control corresponds to an overall tilt of the two plates relative to the shaft of the rotor, owing to said mechanism.
Among the patents or patent applications describing swash plate mechanisms, we may for example cite: EP 0 162 773 A1, WO 2009/010644 A2, etc. Document FR 2 890 375 A1 discloses a rotary wing control assembly comprising at least two coaxial and contra-rotating rotors, each fastened on a rotor shaft and associated with a swash plate, each swash plate consisting of two elements rotating relative to one another, each swash plate comprising one of its rotary elements subject in rotation to the associated rotor and each swash plate further being provided with means allowing its tilt relative to its associated rotor as well as means for transmitting its tilt in the form of lead inclination of the blades of its associated rotor, the control assembly comprising means continuously imposing a same tilt of two swash plates, characterized in that it comprises yaw control means able to impose a translational movement on one of the rotor shafts relative to another rotor shaft of a set of two rotors in order to vary the torque applied to at least one of the rotors.
Furthermore, a helicopter rotor drive system is known, called Direct Turbine Driven Rotor (DTDR), which is for example described in document EP 1 990 275 B 1, U.S. Pat. No. 3,417,825 A, FR 2 532 273 A1, U.S. Pat. No. 4,589,611 A, etc. The system is characterized by the presence of two contra-rotating coaxial primary rotors mounted on a vertical axis: an upper rotor and a lower rotor.
These two rotors are driven by contra-rotating coaxial turbines having no stage fixed to the fuselage. The motor torque of one turbine is then compensated exactly by that of the other turbine.
The corollary to this configuration is that the rotation speeds of the two rotors are not necessarily equal, these speeds being determined by the drag resistance of the blades to the respective motor torques of the turbines of the two rotors, which are equal.
If, for example, the blades of a first rotor have a smaller angle of attack and therefore a smaller drag coefficient than those of the second rotor, this first rotor will rotate faster than the second rotor such that the resistant torques of the two rotors balance one another out.
Experience shows that it is desirable for the two DTDR turbine rotors to have respective rotation speeds as equal as possible, in absolute value. Indeed, the powers of the two turbines are then equal, their performance is optimal and the gyroscopic torques of the two rotors cancel one another out.
Experience also shows that, to achieve this, it is necessary to give approximately 1° more in angle of attack to the blades of the lower rotor relative to the upper rotor when the aircraft is at nearly maximum power, in stationary flight. For example, an angle of attack of 9° will be given for the upper rotor and 10° for the lower rotor.
Conversely, at zero power in autorotation, tests show that the situation is reversed: it is necessary to give about 1° more in angle of attack to the blades of the upper rotor relative to those of the lower rotor. For example, 3° of angle of attack will be given for the upper rotor and 2° for the lower rotor.
These values are indicative and in particular depend on the geometry of the rotor.
These variable angles of attack are managed by the general pitch control: in the example above, there will be a travel of 9°−3°=6° on the general pitch of the upper rotor and a travel of 10°−2°=8° on the general pitch of the lower rotor.
For the intermediate levels of general pitch, a linear variation between the limit values of each rotor is appropriate to retain more or less equal rotation speeds.
Also superimposed on the general pitch, as mentioned above, is a cyclic pitch oscillating around zero, and which gives the blades a variable angle of attack in azimuth and the rotor a control function in pitch and roll. To that end, the swash plate controlling the angle of attack of the blades is mounted on a ball joint that allows it to tilt in order to give cyclic pitch and to translate along the axis of the rotor to give general pitch.
In the present case of the DTDR rotor, it is important that, irrespective of the general pitch lift, the cyclic pitch variation is the same for both rotors, so that these two rotors tilt with the same angle, failing which they would be likely to meet.
The DTDR rotor must therefore have a variable general pitch from one rotor to the other, while having the same cyclic pitch on both rotors.
A few contra-rotating rotor systems are already known, for example the Kamov system, which have a differential general pitch mechanism adjustable during flight from one rotor to the other. This mechanism acts on the distance between the two swash plates via a rod linkage housed inside the mast and makes it possible to differentiate the general pitch of the rotors in order to control the yaw of the aircraft.